1. Field of the Invention
The present invention relates to an automatic equalizer converges in response to a received signal and a data mode convergence method.
2. Description of the Related Art
Modems (modulator/demodulator devices) have been used widely for data transmission over voice-grade analog lines. Modems are provided with automatic equalizers, for example, which correct that distortion of the received signal caused by an analog transmission line and establish synchronization of the received data.
Currently, when a terminal transmits data to another terminal using an analog voice line, the terminal on the receiving side performs a preset of an automatic equalizer of the modem of the local apparatus, a reproduction of a timing and a gain control of a gain control circuit based on the training signal. This enables the automatic equalizer to converge so that the distortion of the data is corrected and data can be demodulated.
In a data transmission system in which a center apparatus and a plurality of terminals are connected to a single transmission line in parallel, called a multi point system, it is necessary to enable the automatic equalizer to converge by making the center apparatus transmit a training signal every time the terminal diverges, when the automatic equalizer of the modem on the terminal loses its equalizing capability, and the automatic equalizer attempts to converge in accordance with the above method at the time of the divergence of the equalizer of the modem on the terminal. Thus, the above method, of enabling the diverged automatic equalizer to converge, decreases the efficiency of the data processing operation in the whole system.
Therefore, the data mode convergence method for enabling the automatic equalizer to converge with respective terminals receiving data has been proposed and some examples are shown in Japanese Patent early disclosure number 56-78243 and Japanese Patent Application No. 1-36284.
The conventional data mode convergence method is explained by referring to FIGS. 1 to 4. In the multi-point system, modems 11 to 13 of a plurality of terminals (slave station) 16 are connected to the modem 10 on the master station, such as center 15 in parallel through an analog voice line. In this system, the master station modem 10 operate at all times and the slave station modems 11 to 13 have their power sources turned on only when necessary, thereby transmitting the data.
The transmission side modulates data received through data transmission utilizing the analog line, it divides the data to be transmitted into groups of predetermined numbers of bits, (for example, three bits in FIG. 2A) and assigns the divided data to n, (for example, eight in FIG. 2A) signal points on a two dimensional plane. The receiving side demodulates the received data by judging which signal point, chosen out of the n signal points, the received signal corresponds to, based on the coordinate data (which are called as receiving on the two dimensional plane). The point in the coordinate data is called the receiving point.
FIG. 3 is a schematic diagram of a conventional automatic equalizer. In the automatic equalizer the decision section 2 determines the target signal point based on the output of the automatic equalizer (EQL) 1. Specifically the coordinate data on the two dimensional plane of the received signal, whose gain is controlled by an AGC controller (described later) in the automatic equalizer 1. An arithmetic unit calculates the difference between the predecision input and postdecision output, thereby obtaining error data. Multiplier 6 multiplies the error data by the signal designating whether the determined signal point is outside the region surrounded by a dotted line in FIG. 2A or inside the region surrounded by the dotted line, (Specifically, "1" if it is outside the region and "0" if it is inside the region.)
If the signal point is located outside the dotted line shown in FIG. 2A, multiplier 6 multiplies error data by "1". When the signal point is located inside the dotted line, the multiplier 6 multiplies the data error data by "0". This is because the signal point inside the dotted line in FIG. 2A has a high possibility of being mistaken since the distances between signal points are short and the amount of interference between the data is large. Thus, the tap coefficients of automatic equalizer 1 are corrected by using error data of the signal points outside the dotted line which have a lower possibility of being mistaken.
Normalized data NOR from the decision section 2 and the predecision output are applied to a carrier automatic phase correcting section (CAPC) 5 where a phase correcting quantity is calculated. The output of the automatic equalizing section is multiplied by the phase correcting quantity by a multiplier 3, thereby performing phase correction. At the same time, the phase correcting quantity and the output of the multiplier 6 are multiplied by a multiplier 7 and the result is output to the automatic equalizing section 1 as a tap correcting quantity.
To enable the automatic equalizer to converge, it is necessary to first adjust the output level of automatic equalizer 1 to the optimum value and then to correct the phase of the output of automatic equalizer 1.
FIG. 4 shows an example of an arrangement of the automatic equalizing section 1. The error output of the multiplier 7 (CAPC error) and center tap data are multiplied by a multiplier 103, and the result is then multiplied by a given control coefficient .alpha. in a multiplier 104. The result of multiplication is output to an AGC 105 as a reference voltage. The AGC 105 controls the level of an equalized output of an adder 102 according to the reference voltage, outputting the resulting tap coefficient to a multiplier 101.
In this way, the automatic equalizer is made convergent while the respective terminals are receiving the data.
However, the conventional data mode convergence method has the following drawbacks.
(1) The algorithm used in the conventional technique has not been established. In order to accommodate various types of lines, it is necessary to determine the control coefficient .alpha. shown in FIG. 4 on the basis of experimental results. However, it is difficult to determine the optimum value by experiment because signal points are made multi-valued, and the level adjustment for convergence is difficult.
(2) In the prior art, it takes time for the automatic equalizer to change from a state in which it does not completely converge, or, more specifically, a state in which the receiving points are spread out and distributed around the signal points as shown in FIG. 2C, to a state in which the automatic equalizer reaches convergence as shown in FIG. 2B when the output level of automatic equalizer 1 is controlled to be constant.
(3) Both the phase error and amplitude error are fed back, and thus, requiring additional time to enable the automatic equalizer to converge.